1. Field of the Invention
The present invention relates to a superconducting coil apparatus, such as a levitating force supplying superconducting on-board coil apparatus for a magnetically levitating train, and a method of manufacturing the coil apparatus.
2. Description of the Related Art
This type of superconducting coil apparatus comprises a cryostat having a racetrack-shaped container, a racetrack-shaped resin-impregnated superconducting coil body contained in the container, and a plurality of members (fixing members) interposed between the superconducting coil body and the cryostat and having a function of allowing a coolant to flow between the superconducting coil body and the inner surface of the cryostat and a function of fixing the superconducting coil body within the cryostat.
By virtue of the fixing members, a very-low-temperature coolant (typically, helium) is let to flow through the space defined by the inner surfaces of the cryostat and the peripheral surfaces of the superconducting coil body and the superconducting coil body is cooled below the superconducting critical temperature.
This type of superconducting coil apparatus is manufactured in the following manner. A superconducting coil body is obtained by solidifying a superconducting coil bundle with a resin. The superconducting coil bundle is obtained by subjecting a superconducting wire or insulating material to predetermined processing. Specifically, the superconducting wire, which is obtained by coating a superconducting core with copper or other stabilizing material, is wound a necessary number of times in a racetrack shape with thin insulating layers interposed, thus forming the superconducting coil bundle. The superconducting coil bundle is impregnated with epoxy resin. By hardening the resultant structure, a composite superconducting coil body is obtained.
The superconducting coil body is contained in a racetrack-shaped space of a cryostat. Thereafter, a plurality of members (fixing members) are interposed between the superconducting coil body and the cryostat. The fixing members has a function of allowing a coolant to flow between the superconducting coil body and the cryostat and a function of fixing the superconducting coil body within the cryostat.
The above-described conventional superconducting coil apparatus has the following problem. In the conventional apparatus, a resin layer of uniform thickness is formed on the surface portion of the superconducting coil body. A part of the resin layer contacts the fixing member, and most the rest contacts the coolant. When the superconducting coil body is energized, electromagnetic force acts on the coil body so as to make the coil body circular. The fixing members and the superconducting coil body tend to be displaced from each other by the electromagnetic force. In this case, even if the degree of displacement is about several-tens of .mu.m, frictional heat occurs at an interface between the fixing members and the coil body. At very low temperatures such as at liquid helium temperature, the specific heat of substances is extremely low. Thus, the generated frictional heat tends to be conducted to the superconducting wire adjacent the resin layer of the surface portion. If the temperature of the superconducting wire is raised to a normal conducting transition temperature by the frictional heat, a quench occurs. In order to prevent frictional heat from being easily transmitted to the superconducting wire, it is necessary to thicken the resin layer constituting the surface portion, thereby increasing the heat resistance and dispersing the heat widely. On the other hand, when the superconducting coil body is energized or deenergized or when the coil body is mounted on a magnetically levitated train, an eddy current loss and hysteresis loss occur and consequently heat occurs in the superconducting coil body. It is necessary to quickly transmit the internally generated heat to the coolant via the resin layer constituting the surface portion. If the temperature of the superconducting wire is raised to the normal conducting transition temperature by the internal heat, a quench occurs. In order to quickly transmit the internal heat, it is necessary to make the resin layer thin and sufficiently reduce the heat resistance of the resin layer.
As can be seen from the above, in order to prevent the quench due to frictional heat, it is necessary to thicken, as much as possible, the resin layer constituting the surface portion. In addition, in order to prevent the quench due to internally generated heat, it is necessary to make the resin layer thin as much as possible. It is therefore necessary to meet these contradictory requirements. In the conventional superconducting coil, the resin layer constituting the surface portion of the superconducting coil body is made to have a uniform thickness. If one requirement is met, the other is not met; both requirements cannot be met.